Independent and combined roles of trace Mg and Ag additions in properties precipitation process and precipitation kinetics of Al–Cu–Li–(Mg)–(Ag)–Zr–Ti alloys
Introduction
In ternary Al–Cu–Mg alloys with high Cu:Mg weight ratios (e.g. 10–20), small additions of Mg (e.g. 0.3 wt%) promote strengthening phases which consist of the tetragonal θ′ phase (Al2Cu) and the orthorhombic S and S′ phases (Al2CuMg)1, 2. In contrast, silver has no effect in binary Al–Cu alloys aged at elevated temperatures[3], but trace additions of Ag (e.g. 0.1 at.%) to Al–Cu–Mg alloys with high Cu:Mg ratios are well known to stimulate the formation of a new phase designated Ω which forms as a thin hexagonal- or orthorhombic-shaped plate-like precipitate on the {111}α planes4, 5, 6, 7, 8, 9. As no Ω was observed in Al–Cu–Ag alloys but was in Al–Cu–Mg–Ag alloys, the existence of Ω was previously considered as a result of a combined effect of Mg and Ag[10]. However, some studies have shown that Ω plates were also present in ternary Al–Cu–Mg alloys free of Ag11, 12, indicating that only small additions of Mg are essential for the precipitation of Ω while Ag serves rather to enhance the precipitation of Ω kinetically at the expense of θ′ phases.
The nucleation mechanism of Ω as well as the roles of Mg and Ag on the nucleation process of Ω are still unknown. Taylor et al.[5] and Auld[13] suggested that Ω nucleated on the precursor precipitate of Mg3Ag which forms on the {111}α planes. Cousland and Tate[14] failed to observe the Mg3Ag phase but found the presence of GP zones which consist of a composition of MgAg as an alternative precursor phase. Such kinds of precursor phases are supported by the recent evidence[15] which has confirmed that the homogeneously dispersed Ω phase was formed exclusively through a continuous structural change of the GP zones on {111}α planes which were rich in Cu, Ag and Mg. Other evidence of a possible precursor to Ω was proposed by Abis et al.16, 17 as a new hexagonal phase called Ω′. With regard to the existing forms and positions of Mg and Ag in Al–Cu–Mg–Ag alloys, Shollock et al.[18] and Grovenor et al.[19] have detected the presence of Mg and Ag inside Ω. Furthermore, both Mg and Ag were detected to segregate to the broad faces of the α/Ω interfaces with significant levels during the early stages of aging and extended aging20, 21, 22.
Nucleation aids such as the elements Mn, Zr, Cd, Ag and Mg can significantly improve the age hardening behavior of Al–Cu–Li alloys. Recently, a family of Weldalite® Al–Cu–Li alloys with minor amounts of Mg and Ag additions have been successfully developed to exhibit excellent properties both with and without prior cold work upon natural and artificial aging[23]. Their ultra-high strength in the peak-aged condition is primarily attributed to the uniform dispersion of T1 phases (Al2CuLi). In Weldalite® 049-type alloys (e.g. 2095), the strengthening phases are identified asAl–Cu GP zone and the δ′ phase (Al3Li) upon natural aging while as T1, θ′ and S′ phases upon artificial aging tempers with T1 as the dominant strengthening phase24, 25, 26, 27, 28, 29.
The T1 phase is similar to Ω in several respects. T1 phases appear as semi-coherent plates on the {111}α planes with the same orientation relationship to the matrix as Ω and has a similar structure to Ω with an approximate 2:1 ratio (c:a)[30]. Due to the similarities of morphologies and the electron diffraction patterns between T1 and Ω, it is difficult to distinguish Ω from T1 other than by high resolution transmission electron microscopy (HRTEM). Based on these facts, Langan and Pickens[24] previously identified the predominant strengthening phase in Weldalite® 049 alloys as the T1-type phase. Recent studies by Herring et al.[31] and Lee et al.[32] have confirmed that the primary strengthening {111}α precipitate is indeed the T1 phase at the relatively high Li range in Al–Cu–Li–Mg–Ag alloys.
Because of the similarities between the T1 and Ω phases, the roles of Mg and Ag on the precipitation process of T1 may be reasonably similar to the way Mg and Ag influence the precipitation of Ω. Some investigations32, 33, 34 have detected the association of Mg and Ag with T1. It has been found that the addition of Mg to Al–Cu–Li–Zr alloy with high Cu:Li ratios (e.g. 3.6[34] and 4.2[35]) had a pronounced effect to induce a refinement of T1 with a high density while Ag addition had no effect on the T1 precipitation process and kinetics.
The recently registered Weldalite® 049-type Al–Cu–Li–Mg–Ag alloy 2195, which has been committed as an essential tank material in NASA’s Lightweight Shuttle Tank Program[36], exhibits great potential for development. Meanwhile, the independent and combined effects of Mg and Ag on the precipitation of Ω and T1 plates as well as their segregation to these plates are still indeterminate. The present study is undertaken in detail to clarify the independent and combined roles of Mg and Ag in properties, precipitation process and precipitation kinetics in alloy 2195 qualitatively and quantitatively.
Section snippets
Experimental procedure
The experimental alloys with the compositions as shown in Table 1 were cast as small 5 kg ingots in an argon atmosphere. The ingots were homogenized, scalped and rolled to 2 mm thick sheets. The specimens were solution heat treated at 504°C for 1 h in a salt bath, then quenched into cold water and subsequently aged at 180°C without prior cold work (T6 temper). The samples for tensile tests were cut from the sheets in the longitudinal direction. A strain rate of 1.0×10−3/S was applied. Thin foil
Tensile properties
Fig. 1 shows the tensile property of the three experimental alloys aged at 180°C with respect to aging time. The start points of the curves indicate that the order of solution-strengthening in the as-quenched condition is Mg+Ag>Mg>Ag. Alloy 2195 exhibits much more rapid and pronounced age-strengthening behavior than alloys M and A. There is an increment of 182 MPa from as-quenched strength value to peak-aging value at 10 h. Alloy 2195 attains a constant and slightly decreasing strength from
Solute–solute interaction and solute–vacancy interaction
Trace additions of particular elements in Al–Cu alloys such as Cd, In, Sn, Ag, Mg and Li can influence the age-hardening performance at room or elevated temperatures through different effects on the nucleation and precipitation process. The difference can be interpreted in terms of two major factors, i.e. solute–solute interaction and solute–vacancy interaction. Table 2 shows extracted values from the appending lists reported by Niessen et al.[60] presenting the calculated enthalpy of solution
Conclusions
- 1.
The Al–4.01Cu–1.11Li–0.19Zr–0.11Ti alloy with independent Mg additions exhibits the significant precipitation of θ′ through favoring GP-zone formation at the early aging stage. Independent Mg additions promote the T1 precipitation, decrease activation energy for the growth of θ′ plates and reduce lengthening rates of T1 and θ′ plates. Some Mg atoms are suggested to segregate at the jogs of T1 and θ′ plates to modify the large misfit energy normal to the habit planes of T1 and θ′ phases.
- 2.
The
Acknowledgements
The authors would like to thank Lij-un Wu, Den-feng Yin and Hui-hui Fang for experimental assistance.
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